The present invention relates to electronic devices, and, more particularly, to semiconductor circuits and methods useful for extracting subband information from broadband digital data streams.
Digital Systems with Down Converters
Communications systems such as broadcast radio use frequency division multiplexing (FDM) to simulaneously transmit differing information signals from several sources in a single locale. Typically, each source modulates its carrier frequency with its information signal and keeps within its allocated frequency band. Extraction of a desired information signal from a received broadband of simultaneous broadcasts may be performed by mixing down (down conversion by the selected carrier frequency) followed by lowpass filtering and demodulation as schematically illustrated by system 100 in FIG. 1. Indeed, system 100 receives radio frequency signals (e.g., 100-200 MHz) at antenna 102, filters and mixes the signals down to intermediate frequencies (e.g., 1-10 MHz) with a wideband tuner 104, converts from analog to digital format with sampling analog-to-digital converter 106, extracts the selected frequency band (e.g., of width 5 KHz) with digital down converter 108 which performs the down conversion and filtering, and demodulates and reconstructs an analog information signal with demodulator/processor 110. For example, if wideband tuner 104 has a 10 MHz output bandwidth, then analog-to-digital converter 106 will sample at 20 MHz or more (at least the Nyquist rate), and digital down converter 108 will output a 5 KHz selected band at a sampling rate of 10 KHz. That is, digital down converter 108 may decrease the sampling rate due to the small bandwidth of its output without loss of information.
The problems of construction of system 100 include realizing digital down converter 108 operating at a high sampling frequency while maintaining a low ripple sharp cutoff filter which has programmable down conversion frequency and programmable bandwidth. Known realizations of a down conversion function include the combination of a numerically controlled oscillator/modulator (NCOM) such as the HSP45106 manufactured by Harris Corporation together with two decimating digital filters (one for the in-phase and one for the quadrature outputs of the NCOM) such as the HSP43220 also manufactured by Harris Corporation. A single chip realization such as the GC1011 digital reciver chip manufactured by Graychip, Inc.
Another example with digital down conversion is a long distance telephone system which may encode multiple voice channels in an FDM format for long distance transmission but convert this to a time division multiplexed (TDM) format for digital switching. FIG. 2a illustrates FDM to TDM system 200 with analog-to-digital converter 202 feeding a bank of digital down converters 204 whose outputs are time multiplexed by trunk formatter 206. FIG. 2b shows the frequency domain operation of system 200 where each of the digital down converters 204 extracts one of the voice channels. Again, the problem of system 200 include realization of a digital down converter with high frequency operation together with low ripple sharp cutoff filter. Multistage finite impluse response filters could be used but involve excessive amounts of hardware for more than minimal precision and frequency range.
Crochiere and Rabiner, Multirate Digital Signal Processing (Prentice-Hall 1983) provides general information regarding signal processing using sampling rate changes. Hogenauer, An Economical Class of Digital Filters for Decimation and Interpolation, 29 IEEE Tr.Ac.Sp.Sig.Proc. 155 (1981) discloses decimation filters made of an integrator section followed by a differencing comb section operating at a low sampling rate.
Features
The present invention provides a digital down converter with a programmable down conversion frequency, programmable bandwidth, programmable output format, serial control word format, and multichip module compatibility.